1 TB = 1024 GB.
2 TB = 2048 GB.
New Super Mario Bros. U is 1.8 GB.
Nintendo Land is is 3.2 GB.
Tekken Tag Tournament 2 Wii U Edition is 16.7 GB.
Wii U discs are up to 25 GB.
A 1 TB HDD can theoretically store 568 copies of NSMBU, 320 copies of Nintendo Land, 61 copies of TTT2, or 40 games at a full 25 GB each.
A 2 TB HDD can theoretically store 1137 copies(!) of NSMBU, 640 copies of Nintendo Land, 122 copies of TTT2, or 81 games at a full 25 GB each.
There is no known disk install feature.
Forget about partitioning. As external HDDs formatted for the Wii U won't be inter-operable with PCs, be sure to think about how many Wii U games you'll be downloading over the course of the generation before you waste money on an excessively large storage device.
See 9:30-10:01 here.
To use an external USB storage device, select Format External USB Storage Device from Data Management in the Settings menu. This will format it for use with the Wii U, a format different from that used by PCs and other devices, on which it will no longer be usable. Please understand ahead of time that a hard disk used as an external USB storage device cannot be used on PCs and other devices.
Originally Posted by Satoru Iwata
USB記録メディアを使うには設定の中の「データ管理」の「USB記録メディアを初期化する」で初期化をすると、使えるようになります。この際にはWii U専用に初期化され、PCなどで使う際と異なるフォーマットで初期化され、PCなどで使用する事が出来なくなります。USB記録メディアとして使用するハードデ ィスクはPCなど他のデバイスと共用出来ない事は予めご了承ください。
Better yet, if you're getting a Deluxe Set, hold off completely and get a cheaper drive down the line when you need it.
Originally Posted by Nintendo Direct 2012.11.14 translation by cvxfreak
In Data Management, you must format the connected media drive. Note that this means you cannot use the drive with a PC.
SD Cards can hold PC data, Wii Virtual Console, Wii Ware, and Wii save data, but cannot be used to save Wii U [games or] game save data.
Couldn't see this mentioned in the thread, but bear Kryder's Law in mind. Disk space increases as years go by, while price stays the same.
Think about how many games you're going to buy in year 1. It may be more cost effective to buy a small drive (say, 40 - 80GB) and then re-buy when you're low on space, especially with the rapidly falling prices of SSDs.